Drider_it

Ok i forget the tech of this so could one of you throw it up for me

a fly in a car the car is going say 80 mph why dont the fly smash the back windshield.. instead of buzzing around.. i know it has to do with the inside pressure and the containment of it all.. but there is a tech reason for it.

for that matter same for a person juming in a jet..

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glytch

I believe it has something to do with the theory of relativity, right? You're only moving at the speed relative to your surroundings (or container). So, relative to the car, the fly isn't moving at all. Same thing with the person in a jet; relative to the person's surroundings, he is not moving.
Let's hope that had any shred of truth to it as it's been many years since I've had a physics class and my brain is a bit hazy. I just hope I didn't make the whole thing up just now.

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stingc

The air that the fly is suspended in is also moving 80 mph, so how could it be any other way?

krwlz

The only way you could make the fly move is during acceleration. And even then, the air is pretty good at accelerating with you, so the fly gets to do whatever he wants.

lyxo

you also have to consider this.
when you're standing in a bus and the car suddenly accelerates, you jerk back and retain your balance by connecting yourself to the bus somehow( through your feet, a pole, or handle)

in the case of the fly, if accelerated quickly enough then it would it the windshield. However, due to the extreme light weight of the fly, its terminal velocity is quite small, so it doesnt create a bloody mess.

mr sticky

Inertia...and remember, to something as insubstantial as a fly, the air is pretty good about transfering it's energy. In a vacuum, the fly goes splat.

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dimbulb

You got the gist of it. But I don't think its relativity that is at work here. You're probably thinking of frames of reference. So if you take the car as a frame of reference, your velocity relative to it is zero.

Or at least thats what i think. Haven't taken a physics class in years....

triad

Yea. you guys got it, another way of looking at it is that the car/fly arn't moving foward at 80mph, but rather the road/world is moving backwards at 80mph and the car is stationary, that'd be relativity at work...

-Triad

KnifeMissile

It's interesting that you say that the fly won't hit the back windshield. Why are you so certain? Do you speak from experience? Have you had a fly stuck in your car?

The fly will, indeed, not hit the back windshield (if it's on the back, isn't it just a window?) but it has little to do with cabin pressure or containment.

The fly won't hit the back windshield as long as the car is moving at a constant velocity. If you were to accelerate, then the fly would, indeed, hit the back windshield just as surely as you would had you not been sitting in your seat (with your seatbelt on, we'd hope).

Here's a possible history as to how this fly got into your moving car. You stepped into your car and the fly surreptitious slipped in with you. Before you start the car, it is moving at a constant velocity (zero), so everything is okay and no one is being squished against anything (well, gravity is pressing you against your seat, but that's not the point).
You start the car and you accelerate to a decent speed. While accelerating, you are being pressed against your seat. You can feel it yourself--the greater the acceleration, the greater the force is pressing down on you. The fly will also be "pushed" towards the back windshield and will hit it if it can't fly in the direction of the car and keep up with it. If you're accelerating slowly, the fly will simply fly (accelerate itself) in the direction of the car and be fine (most people don't go from 0 to 60 in 7.6 seconds). If, however, you accelerate like a race car driver, then the fly will will not be able to fly fast enough and, indeed, hit the back windshield.
After you've reached the speed limit, you stop accelerating and now cruise at a constant speed. You will no longer feel a force on you and neither will the fly (did it ever?). Everyone has already been pushed to this speed and no longer need pushing. Because the fly is moving at the same speed as the car, there's no reason for the fly to hit the back windshield. This is the situation that you described and this is the explanation.

A more detailed explanation is that the car is being pushed forward with a force and it is this force that accelerates the car. Now, of course, you must be accelerated with the car (eventually!) and this is typically done by having the seat of the car push you. That's why you feel the pressure against the seat.
The fly's situation is a little more complicated. Because the fly is flying, it must fly in the direction of the car in order to accelerate with the car or risk being accelerated by having the back windshield push it up to speed.
Despite what I had suggested earlier, the air in the car is a factor for the fly but it is not the major one. The fly will be pushed forward by the air but it can only be pushed as much as air friction can push it. This is much greater for a fly (relatively speaking) than it would be for us, since the fly is so damn light, but it is still only air friction, also known as drag. It is this and only this about the air that pushes the fly. The pressure is only relevant in how it affects air friction.

I hope people have learned something from this. I hope you made it through and I hope I made sense to you...

Jakejake

Do you feel the air streaming past your face at 80mph when driving? No.

KnifeMissile

Therefore, the back window deserves to be called a windshield? I'm not sure what you're trying to say here...

nash

Ah. I was going to say this.

skier

in a vacuum it doesn't matter to a fly what way he is moving.

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saltfish

K, you have to consider the air inside the car to be just like air inside of a sealed vessle. When the vessel/car accellerates, the air inside has no reason to change it's relative position (assuming consistency of mass within the air, throughout the vessel).

Now if you were to compare the density of the fly versus the density of the surrounding air, there is not a considerable difference between these densities. You could almost say that within a certain margin of error that that fly and the surrounding air are equivalent, notice I did say margin of error to cover my ass on this one) Hence the fly would move very little when the vessel begins to accellerate.

Now, the more substantial the accelleration the faster the fly would drift backward toward the rear windshield. As an example, if you have ever used a lab-grade centrifuge, you know that in order to seperate particles suspened in a fluid, longer times and faster spindle speeds are required for fluids that have particles of very simmilar masses. The more similar their masses, the longer the time required to seperate them. If there is a great difference in their masses then a lower speed and shorter time is required.

In conclusion, if the fly's mass were 10x or 100x more than normal we would see a more dramatic effect.

Same thing goes for the rear windshield, if all the windows were open and both windshilds were removed, then the fly would not maintain it's position inside the vehile, and eventually the fly would no longer be inside the car, as it accellerates away.

KnifeMissile

The air has no reason to change it's relative position? Why not? Is air immune to interia? Why?

The fact of the matter is that the air is "pushed" towards the back of the car just like anything else (air is a thing, you know) in the car. You just don't notice because air pressure is about 100KPa, which is significantly more than the extra pressure caused by the accelerating car (easily much less than even a single Pascal). Believe me, if you were to put that air in a centrifuge and spin it fast enough, there would be a vacuum towards the center of spin!

Also, if you think that the fly has approximately the same density as air then you've never swatted a fly in mid-air. It falls quite quickly (although it's terminal velocity is quite slow). If you want to see something with the same density as air, try a poorly inflated helium baloon.

If there were no windshields, the car will most certainly drive away from the fly. The fly doesn't drive along with the car simply because it's in the car, or because the car is sealing it, or that there is air in the sealed car. It's because something pushed it, either it pushed itself (by flying in the direction of the car) or because the back windsheld pushed it forward. As I've said before, the air in the car will have pushed it forward a little bit as the fly approached the back windshield but, because air friction is exactly that--friction, it can never push the fly as fast as the car. This is because friction can only appose movement, so if the fly were moving toward the back windshield, friction can only slow down the movement towards the back windshield (that, and the air in the car is not moving faster than the car but I was hoping to not be that pedantic).

Bigt6909

yeah.. it's like when you see a train moving it's going pretty fast, but if you're on the train, everything and everyone on the train are only moving as fast as they are walking... it's all due to your point of reference.

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gonadman

I believe the physics the govern this situation is Newton's three laws of motion:
1 An object at rest tends to stay at rest unless acted upon by another force
2 An object in motion tends to stay in motion unless acted upon by another force
3 For every action there is an equal and opposite reaction

Of course this is the simplicity behind what KnifeMissl explained so eloquently in his first post. The force is acceleration, which affects everything within the car, dependant upon its mass(Force= mass*acceleration, right?)

Once the car reached a steady cruising speed, all objects within that car will remain in constant motion relative to objects outside the car, but without motion relative to other objects within the car that are travelling at the same speed. Thus we do not feel wind on our face at 80mph within the car, and others on the train appear as normal to fellow passengers even though they are hurtling along at 85mph.

Of course all this changes dramatically when that dog runs in front of your car, and you slam on the brakes. You can be sure that fly will crash into your front windshield along with everything else in the car, with a force that is proportional to its weight. The objects in the car, due to the second law, continue in motion even though the car is slowing due to brake friction. The ultimate force stopping this motion, in this scenario, is the windshield.

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bt8624

Bingo!

dumbnutofak

Why does a ballon filled with helium move left(when turning left) and Right(when turning right) but yet we move opposite....

We don't know...lol it just does.



Oh the wise words of a smart ass.

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KnifeMissile

For the same reason that the balloon rises up instead of falling down.


The words of one who answers rhetorical questions...

Vogad

..the helium ballon is ligther than the medium (air) surrounding it...